Cancer of the colon, or colorectal cancer, is a common cancer in Western countries.
It appears to be related to a diet high in animal proteins and fats and low in fibre.
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There are genetic types of colorectal cancer, but these account for less than five percent of cases.
Colorectal cancer should be suspected in anyone who experiences a change in bowel habits, bleeding from the rectum or dark blood in the stools, loses weight for no reason or is unusually tired.
Treatment of colorectal cancer depends on the stage of the disease, which is classified using Duke’s Staging.
What is cancer of the colon?
Colorectal cancer affects the large bowel and the rectum. The colon or large bowel is about two metres in length. It is linked to the anus by the rectum, a tube approximately 20 cm long. Cancer that develops in these parts of the bowel wall is termed “colonic”, “rectal” or “colorectal”.
Most large bowel cancer, regardless of what has caused it, arises from what are called adenomatous polyps. Adenomatous is a description of the type of cell involved in the cancer. Polyps are visible as outgrowths of the surface (mucosa) of the bowel. However, polyps are commonly found in the large bowels of middle-aged people and less than 1% of them become cancerous.
However, if an adenomatous polyp is detected by visualising the large bowel, it will be cut off biopsied) and examined under the microscope to determine whether or not it is premalignant (has the potential to spread to other parts of the body). The rest of the bowel will also be carefully examined, since polyps seldom occur singly.
What causes colorectal cancer?
Although the direct cause is not known, it is known that diet plays a role in the development of colorectal cancer. The disease occurs more often in upper socio-economic groups who live in urban areas.
Studies in various countries have shown a direct relationship between deaths from colorectal cancer and the consumption of calories, meat protein, and dietary fat and oil, as well as elevations in blood cholesterol levels.
It is possible that eating animal fats increases the proportion of a certain type of bacteria in the gut (called anaerobes because they live without oxygen). This may result in the conversion of normal bile salts – which help in the digestion of fats – into cancer-causing agents. However, this theory is still highly controversial.
It appears that a diet high in roughage, which produces more frequent, bulkier stools, has a protective effect against colorectal cancer. Populations with a high-fibre diet have a lower incidence of colorectal cancer than populations who eat less fibre. There is a theory that this increase in fibre reduces the amount of time digested food remains in the gut, thus diluting the effect of any cancer-causing agents. But again, this theory is still under a lot of scientific scrutiny.
So, while diet is known to be important in the development of colorectal cancer, there is no single causative agent known.
Who gets colorectal cancer and who is at risk?
Colorectal cancer is a common cancer in Western society, and is becoming more common in certain third world socio-economic groups that have become more westernised. In the United States, colorectal cancer is second only to lung cancer as a cause of cancer death. In the past 40 years the incidence and mortality has not changed substantially in men, although it has declined slightly in women. It generally affects people older than the age of 50.
Risk factors for colorectal cancer
Diet
The so-called Western diet, high in fats and proteins and low in carbohydrates and fibre, seems to be an important risk factor for colorectal cancer. It is a disease of upper socio-economic groups in urban areas. Populations which migrate from countries with a low incidence of this cancer tend to take on the incidence of their adopted country.
Hereditary factors
Up to 25% of people with colorectal cancer have a family history of the disease. These inherited large bowel cancers can be divided into two groups:
Polyposis coli, also called familial polypotic colorectal cancer (FAP). This rare condition is characterised by thousands of polyps throughout the large bowel. These polyps are rarely present before puberty, but are usually present by the age of 25. If left untreated, colorectal cancer will occur in almost all patients by the age of 40. Once these polyps have been detected, patients should have a total colectomy (removal of the colon). Children of people with polyposis coli have a 50% risk of the eventual development of the premalignant syndrome and should be screened annually until they are 35.
Non-polyposis syndrome, or hereditary non-polypotic colorectal cancer (HNPCC). There seems to be clusters of people who have a predisposition to colorectal cancer with no evidence of multiple, colonic polyps. These individuals may have a risk as high as 50% for the development of colorectal cancer. Such families often have a history of multiple primary cancers - frequently, ovarian and endometrial. This population develops the cancers younger, usually in their mid-30s to 40s. Children of these people should also be screened regularly from their early to mid-20s.
Inflammatory bowel disease
Long-standing inflammatory bowel disease, particularly ulcerative colitis, seems to be associated with the subsequent development of colorectal cancer. The risk seems to be relatively small during the first 10 years of the disease, but then appears to increase by about 0.5 to 1% per year. The risk is generally thought to be highest in young patients in whom the whole bowel is affected. Because the symptoms of inflammatory bowel disease are similar to those of bowel cancer (bloody diarrhoea, abdominal cramping and obstruction), ordinary cancer surveillance is not much use. Many experts will recommend colectomy to prevent cancer from developing, particularly in younger patients.
Anal sex and sexually transmitted diseases present in the anal area are a risk factor - for rectal cancer in particular.
Symptoms and signs of colorectal cancer
These vary according to where the tumour is. Lesions higher up in the bowel will often present with chronic, concealed blood loss leading to anaemia (reduced blood haemoglobin), so the presenting symptoms will be tiredness, heart palpitations and angina (chest pain) if the person is sufficiently anaemic. Any middle-aged man who is found to be anaemic must be screened for cancer of the bowel.
If the cancer is lower down in the bowel the symptoms are more noticeable, namely a change in bowel habits, which may be either constipation or diarrhoea, bloody or not. There may also be abdominal cramping.
Rectal cancers will generally present with pain and bleeding.
An advanced cancer will often present with small or large bowel obstruction.
The following symptoms should always be investigated:
Change in regular bowel habit
Blood in the stools (bright red or very dark). Black stools (i.e. that contain changed blood) are also referred to as “malaena”.
Abdominal discomfort, including colic, bloating and fullness.
Vomiting
Weight loss
Fatigue
How is colorectal cancer diagnosed?
If a patient has suspicious symptoms, the doctor will undertake a general medical examination, including feeling for enlarged glands and abnormalities in the abdomen. A rectal examination, by means of a gloved and lubricated finger passed through the anus, is part of the clinical examination.
A stool specimen is usually tested for the presence of concealed (“occult”) blood. A positive, occult stool test could be indicative of underlying colon cancer.
If there is a suspicion of colorectal cancer, the following special investigations may be carried out:
Sigmoidoscopy, which involves passing a lighted tube (sigmoidoscope) through the anus into the rectum and lower colon. In this way, polyps or a cancer can be seen by the doctor. About 65% of cancers of the colon and rectum are within reach of the flexible fibre-optic sigmoidoscope.
Colonoscopy is an extended version of sigmoidoscopy, in which the interior of the whole colon can be seen.
Barium enema, in which x-rays of the colon and rectum are obtained after these are highlighted by means of an enema containing a solution of barium.
A biopsy may be done if a polyp or suspicious region of the inside of the bowel is seen during colonoscopy or sigmoidoscopy.
Can colorectal cancer be prevented?
Familial colorectal cancer
The forms of colorectal cancer which run in families (polypotic and non-polypotic) represent only a small proportion (about 5%) of all cancers of this type. In these families, it is important that potentially affected persons should have regular check-ups. These may include routine investigations for blood in the stools, together with colonoscopy.
The recent identification of the genes that lead to familial polypotic (FAP) and hereditary non-polypotic colorectal cancer (HNPCC) has the potential to revolutionise medical surveillance. In high-risk families, it is now possible to undertake genetic tests (DNA studies) on small blood specimens from potentially affected persons, thereby avoiding the inconvenience of repeated colonoscopy. If a person is shown to have the determinant gene, a higher level of regular check-up is indicated. The value of prophylactic surgical removal of part of the bowel has not yet been established.
It must be emphasised that this molecular approach in South Africa is very new, only appropriate in limited circumstances, and only available at specialised centres. Equally, more than 95% of colorectal cancers do not run in families, and in these instances gene tests are not currently helpful in pre-symptomatic or early diagnosis.
Non-familial colorectal cancer
The cause of the majority of colorectal cancers is unknown. It has been suggested that environmental factors (which can be avoided) include a diet that is high in fat, protein and calories, but low in fibre. Smoking and lack of regular physical exercise have also been implicated.
How is colorectal cancer treated?
Treatment depends on the stage of the disease. Colorectal cancer is a progressive disease and spreads by direct extension through the gut wall, through the blood and through the lymph nodes.
Colorectal cancer is staged in the following way, known as Duke’s staging:
Stage A – the cancer is limited to the wall of the bowel.
Stage B – the cancer extends through the bowel wall.
Stage C – the cancer has spread to the nearby lymph nodes.
Stage D – distant spread (metastases) to the rest of the body, for example lung and liver.
Once the diagnosis has been confirmed and the stage of the colorectal cancer has been determined, appropriate treatment is planned. A team approach, using the combined experience of medical specialists, including gastroenterologists, surgeons and oncologists, is often employed.
Surgery
Whenever possible, the involved segment of bowel is removed surgically, together with adjacent lymph nodes. It is usually feasible for the surgeon to re-connect the healthy bowel but it is sometimes necessary for a surgical opening (colostomy) to be made in the abdominal wall. The purpose of the colostomy is to lead the body’s waste products directly to the outside, rather than via the normal route through the anus. A special bag is worn over the opening, in order to collect the faeces. Advanced technology underlies the construction and attachment of colostomy bags, and the inconvenience to the patient has thereby been minimised.
Pre-operative radiotherapy
Persons with more distal cancers benefit from pre-operative radiotherapy. This reduces the chances of local recurrence after surgical resection. Fractionated radiotherapy (i.e. administered in stages) is given by an oncologist in these cases before colonic surgery, over a period of three to four weeks. In most cases the treatment is well tolerated.
Chemotherapy
Medical treatment may be given to control or destroy the cancer cells (chemotherapy). Various medicines are available, some in the form of pills, while others are injected into a blood vessel. Chemotherapy may be used in combination with surgery.
Immunotherapy
Medicines that stimulate the immune system to combat the cancer (immunotherapy) may be given by intravenous injection. This type of treatment is often used after surgery.
The various forms of treatment of colorectal cancer, either alone or in combination, can all produce side effects. These vary in severity, but represent a further reason for a medical team approach to the care of the affected persons. In the same way, regular check-ups and after-care are an essential aspect of overall medical management.
Follow-up:
Following colonic resection for cancer, life-long assessment will be needed to identify local recurrence or distant metastases (spread). The presence thereof can modify further treatment strategy. Radiotherapy may be needed in persons with bone spread. In some cases, further interval (staged) surgery may be needed to control local recurrence. The follow-up tests needed on an annual basis for at least 5 years include:
Liver metastases (recurrence) are managed with chemotherapy and local resection in the case of a single malignant deposit. The prognosis in such cases can be considerably improved by a combined approach.
What is the outcome of colorectal cancer?
The outcome or prognosis depends largely on the stage at which the cancer was first diagnosed.
Duke’s staging offers an approximate five-year survival percentage for each stage:
Stage A – more than 90% five-year survival
Stage B – 70 to 85% five-year survival
Stage C – 30-60% five-year survival
Stage D – 5% five-year survival
However, these are all approximations based on clinical studies. Individual people may vary widely in their response to different stages of the cancer and different treatments.
When to see your doctor
Consult your doctor if you experience any of the symptoms of colorectal cancer (see "symptoms and signs"), particularly if you are older than 40 and/or have a family history of colorectal cancer:
Any middle-aged man who is found to be anaemic must be investigated for colorectal cancer.
Reviewed by Prof Don du Toit (M.B.Ch.B) (D.Phil.) (Ph.D) (FCS) (FRCS).
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